Salt Byproduct Separation During Formation of Polyarylene Sulfide

ABSTRACT

Methods of forming a polyarylene sulfide and systems as may be utilized in carrying out the methods are described. Included in the formation method is a filtration process for treatment of a mixture, the mixture including a polyarylene sulfide, a salt byproduct of the polyarylene sulfide formation reaction, and a solvent. The filtration process includes maintaining the downstream side of the filter medium at an increased pressure. The downstream pressure can such that the boiling temperature of the mixture at the downstream pressure can be higher than the temperature at which the polyarylene sulfide is insoluble in the solvent.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims filing benefit of U.S. Provisional Patentapplication 61/882,332 having a filing date of Sep. 25, 2013; and UnitedStates Provisional Patent application 62/033,289 having a filing date ofAug. 5, 2014; both of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

Polyarylene sulfides are high-performance polymers that may withstandhigh thermal, chemical, and mechanical stresses and are beneficiallyutilized in a wide variety of applications. Polyarylene sulfides aregenerally formed via polymerization of a dihaloaromatic compound with analkali metal sulfide or an alkali metal hydrosulfide in an organic amidesolvent. Salts are formed as byproducts in the polymerization reaction,and these salt byproducts must be removed to obtain a final productexhibiting desirable traits.

Standard methods for removal of salt byproducts have included theutilization of screens or sieves that rely on a difference in particlesize between the polyarylene sulfide and the salt byproduct.Unfortunately, these methods have proven less than ideal due to loss ofproduct in the form of polyarylene sulfide fines as well as undesirablelevels of salt byproduct remaining trapped in the product polyarylenesulfide granules. Water extraction has been utilized in an attempt toremove salt remaining after the screening/sieving operation, but thisadds an additional step as well as associated costs to the to theformation process and does not solve the problem of the loss ofpolyarylene sulfide fines during the screening/sieving operation.

Solid/liquid extraction processes have also been utilized to remove saltbyproducts from the polymerization product. While fairly effective,extraction methods require a large amount of water and thus create bothwaste and additional operational costs. Other separation processes suchas flashing of solvent followed by sieving and/or water extraction havebeen utilized, but these methods, similar to others, add costs,additional process steps, and undesirable waste to the formationprocess.

What is needed in the art is a method for removal of salt byproductsduring formation of a polyarylene sulfide polymer that can keep capitalcosts low and avoid the formation of additional waste.

SUMMARY OF THE INVENTION

A method of forming a polyarylene sulfide polymer is disclosed. Forinstance, a method can include reacting a dihaloaromatic compound and analkali metal sulfide or an alkali metal hydrosulfide in an organic amidesolvent to form a polyarylene sulfide polymer and a salt. A mixtureincluding the polyarylene sulfide, the salt, and the organic amidesolvent can then be subjected to a filtration process. In the filtrationprocess, the mixture flows to a filter medium from upstream and afiltrate flows away from the filter medium downstream while the salt isretained at the filter medium and forms a salt cake. The downstreampressure of the filtration process can be greater than atmosphericpressure and the boiling temperature of the mixture at the downstreampressure can be greater than the minimum temperature at which thepolyarylene sulfide is fully soluble in the solvent. The downstreampressure thus serves to maintain the polymer in solution during thefiltration and the filtration can be carried out in a temperature rangethat is less than the boiling temperature of the mixture at thedownstream pressure and greater than the minimum temperature at whichthe polyarylene sulfide is fully soluble in the solvent.

In one embodiment, the reaction can be a first stage polymerizationreaction of a multi-stage polyarylene sulfide formation process and thepolyarylene sulfide can be of relatively low molecular weight at thefiltration. In this embodiment the method can also include a secondstage polymerization reaction during which the molecular weight of thepolyarylene sulfide can be increased.

BRIEF DESCRIPTION OF THE FIGURES

The present disclosure may be better understood with reference to thefollowing figures:

FIG. 1 is a flow diagram for a polyarylene sulfide formation processincluding a salt byproduct separation process as described herein.

FIG. 2 illustrates filter media following a salt byproduct separationprocess with atmospheric downstream pressure during the filtration.

FIG. 3 illustrates filter media following a salt byproduct separationprocess in which the downstream pressure was maintained such that theboiling temperature of the filtrate at the downstream pressure wasgreater than the temperature at which the polyarylene sulfide polymerwas fully soluble in the solvent.

DETAILED DESCRIPTION

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

The present disclosure is generally directed to methods of forming apolyarylene sulfide and systems as may be utilized in carrying out themethods. More specifically, disclosed is a filtration process fortreatment of a mixture, the mixture including a polyarylene sulfide, asalt byproduct of the polyarylene sulfide formation reaction, and asolvent. The filtration process includes maintaining the downstream sideof the filter medium at an increased pressure. The downstream pressurecan such that the boiling temperature of the mixture at the downstreampressure can be greater than the minimum temperature at which thepolyarylene sulfide is fully soluble in the solvent. The filtration canbe carried out within a temperature range between the boilingtemperature of the mixture and the temperature at which the polymer isfully soluble in the solvent. The downstream pressure thus serves tomaintain the polymer in solution during the filtration.

During the filtration process, the pressure differential across thefilter medium can be static or can vary throughout the process. In anycase, the pressure differential across the filter medium can becontrolled so as to ensure that the filtration process can proceed at areasonable rate. The filtration process can remove salt byproduct fromthe mixture according to a simple, fast, and reliable method that canutilize existing filtration equipment technology at lower capital costs.In addition, the method can utilize less solvent in a final wash of thepolyarylene sulfide, which can reduce waste produced during thepolyarylene sulfide formation process.

Through utilization of the filtration process, most or all of the saltbyproduct formed during the polymerization reaction can be removed withlittle or no precipitation of the polyarylene sulfide within thefiltration unit. Thus, the filtration process can lead to longer filterlife and less production downtime as well as a decrease in wasteproduction as waste-generating extraction operations necessary in thepast can be decreased or eliminated. Moreover, the method can eliminatethe need for a sieving process that relies on size differential betweenthe polyarylene sulfide granules and the salt byproduct.

According to one embodiment, the filtration process can be carried outfollowing a polymerization reaction during which the polyarylene sulfidethat is formed is a relatively low molecular weight prepolymer and priorto a second stage polymerization reaction during which the molecularweight of the polyarylene sulfide is increased to reach a commerciallyuseful value. By separating the salt byproduct prior to a second stagepolymerization reaction, further improvements in a formation process canbe realized, including an increase in the reaction rate of the secondstage reaction as a lower solvent to sulfur ratio may be used in thesecond stage, effectively increasing the polymer concentration andformation rate. In addition, by carrying out the salt separation processprior to the second stage polymerization reaction, the capacity of thereactor for the second stage can be increased due to removal of the saltfrom the mixture to be charged to a second stage reactor.

This is not a requirement of the filtration process, however, and inother embodiments the filtration process can be carried out following apolymerization reaction during which the polyarylene sulfide that isformed is a higher molecular weight polymer. For instance, thefiltration process can be carried out following a single polymerizationprocess or following a second (or subsequent) polymerization stage in amulti-stage polymerization process.

The formation process for producing the polyarylene sulfide can includereacting a compound that provides a hydrosulfide ion, e.g., an alkalimetal sulfide or an alkali metal hydrosulfide, with a dihaloaromaticcompound in an organic amide solvent. One embodiment for forming thepolyarylene sulfide is illustrated in FIG. 1. According to thisembodiment, a first stage polymerization reaction can be carried out inwhich monomers are reacted in the first stage to form a polyarylenesulfide prepolymer of relatively low molecular weight.

In general, a polyarylene sulfide as may be formed according to theprocess can be a polyarylene thioether containing repeat units of theformula (I):

—[(Ar¹)_(n)—X]_(m)—[(A²)_(i)Y]_(j)—[(Ar³)_(k)—Z]_(l)[(Ar⁴)_(o)—W]_(p)—  (I)

wherein Ar¹, Ar², Ar³, and Ar⁴ are the same or different and are aryleneunits of 6 to 18 carbon atoms; W, X, Y, and Z are the same or differentand are bivalent linking groups selected from —SO₂—, —S—, —SO—, —CO—,—O—, —COO— or alkylene or alkylidene groups of 1 to 6 carbon atoms andwherein at least one of the linking groups is —S—; and n, m, i, j, k, l,o, and p are independently zero or 1, 2, 3, or 4, subject to the provisothat their sum total is not less than 2. The arylene units Ar¹, Ar²,Ar³, and Ar⁴ may be selectively substituted or unsubstituted.Advantageous arylene systems are phenylene, biphenylene, naphthylene,anthracene and phenanthrene. The polyarylene sulfide can typicallyinclude more than about 30 mol %, more than about 50 mol %, or more thanabout 70 mol % arylene sulfide (—AR—S—) units. In one embodiment thepolyarylene sulfide includes at least 85 mol % sulfide linkages attacheddirectly to two aromatic rings.

In one embodiment, the polyarylene sulfide formed by the process can bea polyphenylene sulfide, defined herein as containing the phenylenesulfide structure —(C₆H₄—S)_(n)— (wherein n is an integer of 1 or more)as a component thereof.

The monomers utilized in forming the polyarylene sulfide can include analkali metal sulfide that can be, for example, lithium sulfide, sodiumsulfide, potassium sulfide, rubidium sulfide, cesium sulfide or amixture thereof. When the alkali metal sulfide is a hydrate or anaqueous mixture, the alkali metal sulfide can be processed according toa dehydrating operation in advance of the polymerization reaction. Analkali metal sulfide can also be generated in situ. In addition, a smallamount of an alkali metal hydroxide can be included in the reaction toremove or react impurities (e.g., to change such impurities to harmlessmaterials) such as an alkali metal polysulfide or an alkali metalthiosulfate, which may be present in a very small amount with the alkalimetal sulfide.

A dihaloaromatic compound can be charged to the first stagepolymerization reaction in conjunction with the alkali metal sulfide. Adihaloaromatic monomer can be, without limitation, o-dihalobenzene,m-dihalobenzene, p-dihalobenzene, dihalotoluene, dihalonaphthalene,methoxy-dihalobenzene, dihalobiphenyl, dihalobenzoic acid,dihalodiphenyl ether, dihalodiphenyl sulfone, dihalodiphenyl sulfoxideor dihalodiphenyl ketone. Dihaloaromatic compounds may be used eithersingly or in any combination thereof. Specific exemplary dihaloaromaticcompounds can include, without limitation, p-dichlorobenzene;m-dichlorobenzene; o-dichlorobenzene: 2,5-dichlorotoluene;1,4-dibromobenzene; 1,4-dichloronaphthalene;1-methoxy-2,5-dichlorobenzene; 4,4′-dichlorobiphenyl;3,5-dichlorobenzoic acid; 4,4′-dichlorodiphenyl ether;4,4′-dichlorodiphenylsulfone; 4,4′-dichlorodiphenylsulfoxide; and4,4′-dichlorodiphenyl ketone.

The halogen atom of the dihaloaromatic compound can be fluorine,chlorine, bromine or iodine, and two halogen atoms in the samedihaloaromatic compound may be the same or different from each other. Inone embodiment, o-dichlorobenzene, m-dichlorobenzene, p-dichlorobenzeneor a mixture of 2 or more compounds thereof is used as thedihalo-aromatic compound.

As is known in the art, it is also possible to use a monohalo compound(not necessarily an aromatic compound) in combination with thedihaloaromatic compound in order to form end groups of the polyarylenesulfide or to regulate the polymerization reaction and/or the molecularweight of the polyarylene sulfide.

The polyarylene sulfide may be a homopolymer or may be a copolymer. By asuitable, selective combination of dihaloaromatic monomers, apolyarylene sulfide copolymer can be formed containing not less than twodifferent units. For instance, in the case where p-dichlorobenzene isused in combination with m-dichlorobenzene or4,4′-dichlorodiphenylsulfone, a polyarylene sulfide copolymer can beformed containing segments having the structure of formula (II):

and segments having the structure of formula (III):

or segments having the structure of formula (IV):

In general, the amount of the dihaloaromatic compound(s) per mole of theeffective amount of the charged alkali metal sulfide can be from 1.0 to2.0 moles, from 1.05 to 2.0 moles, or from 1.1 to 1.7 moles. Thus, thepolyarylene sulfide can include alkyl halide (e.g., alkyl chloride) endgroups.

In another embodiment, a copolymer can be formed and a monomer can becharged to the system having the formula (V):

where the radicals R¹ and R², independently of one another, are ahydrogen, fluorine, chlorine or bromine atom or a branched or unbranchedalkyl or alkoxy radical having from 1 to 6 carbon atoms. In oneembodiment, a monomer of formula (V) can be p-hydroxybenzoic acid or oneof its derivatives

Another monomer as may be charged to the system can have a structure ofthe formula (VI):

One example of a monomer of formula (VI) is2-hydroxynaphthalene-6-carboxylic acid. Monomers of the formula V and VImay be both charged to the system to form a polyarylene sulfidecopolymer.

A polyarylene sulfide copolymer can include a segment derived from apolyarylene sulfide structure of the formula (VII):

Ar—S_(q)  (VII)

where Ar is an aromatic radical, or more than one condensed aromaticradical, and q is a number from 2 to 100, in particular from 5 to 20.The radical Ar in formula (VII) may be a phenylene or naphthyleneradical. In one embodiment, the second segment may be derived frompoly(m-thiophenylene), from poly(o-thiophenylene), or frompoly(p-thiophenylene).

The polyarylene sulfide may be linear, semi-linear, branched orcrosslinked. A linear polyarylene sulfide includes as the mainconstituting unit the repeating unit of —(Ar—S)—. In general, a linearpolyarylene sulfide may include about 80 mol % or more of this repeatingunit. A linear polyarylene sulfide may include a small amount of abranching unit or a cross-linking unit, but the amount of branching orcross-linking units may be less than about 1 mol % of the total monomerunits of the polyarylene sulfide. A linear polyarylene sulfide polymermay be a random copolymer or a block copolymer containing theabove-mentioned repeating unit.

A semi-linear polyarylene sulfide may be formed that may have across-linking structure or a branched structure provided by introducinginto the polymer a small amount of one or more monomers having three ormore reactive functional groups. For instance between about 1 mol % andabout 10 mol % of the polymer may be formed from monomers having threeor more reactive functional groups. Methods that may be used in makingsemi-linear polyarylene sulfide are generally known in the art. By wayof example, monomer components used in forming a semi-linear polyarylenesulfide can include an amount of polyhaloaromatic compounds having 2 ormore halogen substituents per molecule which can be utilized inpreparing branched polymers. Such monomers can be represented by theformula R′X_(n), where each X is selected from chlorine, bromine, andiodine, n is an integer of 3 to 6, and R′ is a polyvalent aromaticradical of valence n which can have up to about 4 methyl substituents,the total number of carbon atoms in R′ being within the range of 6 toabout 16. Examples of some polyhaloaromatic compounds having more thantwo halogens substituted per molecule that can be employed in forming asemi-linear starting polyarylene sulfide include 1,2,3-trichlorobenzene,1,2,4-trichlorobenzene, 1,3-dichloro-5-bromobenzene,1,2,4-triiodobenzene, 1,2,3,5-tetrabromobenzene, hexachlorobenzene,1,3,5-trichloro-2,4,6-trimethylbenzene, 2,2′,4,4′-tetrachlorobiphenyl,2,2′,5,5′-tetra-iodobiphenyl,2,2′,6,6′-tetrabromo-3,3′,5,5′-tetramethylbiphenyl,1,2,3,4-tetrachloronaphthalene, 1,2,4-tribromo-6-methylnaphthalene, andthe like, and mixtures thereof.

Exemplary organic amide solvents used in a forming the polyarylenesulfide can include, without limitation, N-methyl-2-pyrrolidone;N-ethyl-2-pyrrolidone; N,N-dimethylformamide; N,N-dimethylacetamide;N-methylcaprolactam; tetramethylurea; dimethylimidazolidinone;hexamethyl phosphoric acid triamide and mixtures thereof. The amount ofthe organic amide solvent used in the reaction can be, e.g., from 0.2 to5 kilograms per mole (kg/mol) of the effective amount of the alkalimetal sulfide.

During an initial stage of the polymerization, a complex including analkali metal organic amine and an alkali metal hydrogen sulfide can beformed by combining a pre-determined amount of an organic amide solventwith an alkali metal sulfide, an alkali metal hydroxide, and water. Forexample, a complex of sodium methylaminobutyrate (SMAB) and sodiumhydrogen sulfide (NaSH) can be formed by combining a pre-determinedamount of alkali sulfide salt, N-methylpyrrolidone (NMP), water andsodium hydroxide to form a mixture. The mixture can be subjected toheating until the temperature of the mixture has reached from about 200°C. to about 210° C. During this reaction, a combination of water, NMPand some H₂S can be collected as a distillate. The distillate can beanalyzed, e.g., by chromatography, to determine the composition of themixture during the first stage of polymerization.

When carrying out this stage of the polymerization reaction, the alkalimetal sulfide, which usually includes water, can be charged into theorganic amide solvent and the mixture can be heated to distill theexcessive water out of the reaction system. At that time, a part of thealkali metal sulfide will decompose to form an alkali and hydrogensulfide (H₂S). From the generated amount of H₂S, the effective amount ofthe charged alkali metal sulfide can be calculated.

The termination of this initial stage of the polymerization reactionduring which the complex is formed is generally that point at which theconversion rate of the dihaloaromatic compound in the reaction systemreaches not less than about 50 mol %, not less than about 70 mol %, ornot less than about 90 mol % of the theoretical conversion. Thetheoretical conversion of the dihaloaromatic compound can be calculatedfrom one of the following formulas:

(a) In the case wherein the dihaloaromatic compound has been added inexcess (by molar ratio) of the alkali metal sulfide:

${{Conversion}\mspace{14mu} {rate}} = {\frac{X - Y}{X - Z} \times 100}$

(b) In the cases other than (a):

${{Conversion}\mspace{14mu} {rate}} = {\frac{X - Y}{X} \times 100}$

wherein X is the amount of the charged dihaloaromatic monomer; Y is theremaining amount of the dihaloaromatic monomer and Z is the excessiveamount of dihaloaromatic monomer in moles.

(c) In the case other than (a) or (b)

Conversion rate=A/B*100

Wherein A is the total weight of salt collected after removal of theresidual polymer and other species other than salt by-product; B is thetheoretical weight of salt which is two times the molar amount of theeffective sulfide present during the polymerization.

Following formation of the complex, a mixture including the complex isreacted with a dihaloaromatic monomer, for example at a temperature offrom about 180° C. to about 280° C., or from about 200° C. to about 260°C., and the low molecular weight prepolymer can be formed. This initialpolymerization can continue until the conversion rate of thedihaloaromatic compound attains to not less than about 50 mol % of thetheoretically necessary amount.

Following this polymerization reaction, the mean molar mass of theprepolymer as expressed by the weight average molecular weight, M_(W),can be from about 500 g/mol to about 30,000 g/mol, from about 1000 g/molto about 20,000 g/mol, or from about 2000 g/mol to about 15,000 g/mol.

The polymerization reaction apparatus is not especially limited,although it is typically desired to employ an apparatus that is commonlyused in formation of high viscosity fluids. Examples of such a reactionapparatus may include a stirring tank type polymerization reactionapparatus having a stirring device that has a variously shaped stirringblade, such as an anchor type, a multistage type, a spiral-ribbon type,a screw shaft type and the like, or a modified shape thereof. Furtherexamples of such a reaction apparatus include a mixing apparatuscommonly used in kneading, such as a kneader, a roll mill, a Banburymixer, etc.

Following the polymerization reaction and formation of the low molecularweight prepolymer, the mixture can include the prepolymer, the solvent,and one or more salts. For instance, the molar ratio of the solvent tosulfide can be from about 1 to about 10, or from about 2 to about 5. Theproportion by volume of salt formed as a byproduct to the reaction canbe from about 0.05 to about 0.25, or from about 0.1 to about 0.2.

Salts included in the reaction mixture can include those formed as abyproduct during the reaction as well as salts added to the reactionmixture, for instance as a reaction promoter. The salts can be organicor inorganic, i.e. can consist of any combination of organic orinorganic cations with organic or inorganic anions. They can be at leastpartially insoluble in the reaction medium and have a density differentfrom that of the liquid reaction mixture. Typical representatives of theinorganic salts are the halides of alkali or alkaline earth metals whichcan be formed as byproduct of the polymerization reaction(s).Representatives of organic salts can include carboxylates of the alkalimetals, of the alkaline earth metals, of ammonium or of organicallysubstituted ammonium cations which can be used as promoters in thepreparation of the polyarylene sulfide. As utilized herein, carboxylatesrefer to the solids of aliphatic carboxylic acids, e.g. acetic acid orpropionic acid, or aromatic carboxylic acids, for example benzoic acid,and also solids of polyfunctional carboxylic acids.

Precipitation of polyarylene sulfide often occurs at atmosphericpressure at temperatures of about 220° C. to about 230° C. for mixtureshaving a solids content of between about 20 wt. % and about 30 wt. %,which is typical for a polyarylene sulfide prepolymer formation. Whilethe boiling temperature of the solvent can vary, it is often in therange of from about 200° C. to about 220° C. For instance, the boilingtemperature of N-methyl-2-pyrrolidone is about 202° C. at atmosphericpressure. When conditions are such that the boiling temperature of themixture is less than the minimum temperature at which the polyarylenesulfide is fully soluble in the solvent, precipitation of the polymercan occur at a temperature between these two temperatures. This canoccur for any solvent-containing mixture that includes the polymer.However, the boiling temperature of the mixture will increase atincreased pressure. Thus, in the hot filtration process describedherein, the pressure throughout the filtration process (i.e., bothupstream and downstream of the filter) is increased above atmospheric,and the filtration process is carried out at less than the boilingtemperature of the mixture at the minimum pressure of the filtrationprocess (e.g., the downstream pressure) and at greater than the minimumtemperature at which the polymer is fully soluble in the solvent. Thedifference between the boiling temperature of the mixture at the minimumpressure of the filtration process and the minimum temperature at whichthe polymer is fully soluble in the solvent can provide a temperaturerange in which the filtration process can be carried out. For instance,the difference between these two temperatures can be about 10° C. orgreater in some embodiments, or about 15° C. or greater in someembodiments, for example from about 10° C. to about 50° C. in someembodiments.

With reference to FIG. 1, the filtration process can include afiltration unit 100 in which the pressure downstream of the filtermedium 110 (e.g., at the outlet as illustrated in FIG. 1) is such thatthe boiling temperature of the mixture at this downstream pressure isgreater than the minimum temperature at which the polyarylene sulfide isfully soluble in the solvent. For instance, the downstream pressure canbe greater than about 300 kilopascals (kPa). In some embodiments, thedownstream pressure can be from about 300 kPa to about 1200 kPa, fromabout 400 kPa to about 1100 kPa, or from about 500 kPa to about 1000kPa.

In addition to increased downstream pressure, the filtration process canbe carried out so as to maintain a positive pressure differential acrossthe filter medium 110 so as to provide a desired flow rate across thefilter medium 110 and an efficient filtration process. For instance, thepressure difference across the filter medium 110 (e.g., from the inletto the outlet as illustrated in FIG. 1) for at least a portion of thefiltration process can be from about 30 kPa to about 500 kPa, from about50 kPa to about 400 kPa, or from 70 kPa to about 300 kPa.

The mixture and filtrate can be heated throughout the filtration processso as to prevent precipitation of the polyarylene sulfide. For instance,the mixture and filtrate can be at a temperature of from about 220° C.to about 300° C. Moreover, the mixture and the filtrate can be at thesame or different temperatures from one another. In addition, the filterunit 100 can be heated to a temperature at or near that of the mixtureand/or the filtrate, so as to ensure the maintenance of the increasedtemperature of the materials and avoid undesirable precipitation of thepolyarylene sulfide at any point within the filter unit, i.e., not onlyon the filter medium 110, but also throughout the unit 100. Forinstance, at least that portion of the filter unit 100 that will contactthe mixture and the filtrate can be held at a temperature of betweenabout 220° C. and about 300° C.

The upstream and downstream pressure as well as the pressuredifferential between the two can remain static or can vary throughoutthe filtration process as long as two basic criteria are met. These twocriteria are 1) the downstream pressure is greater than the minimumpressure of the filtration process (i.e., that pressure at which theboiling temperature of the mixture at this downstream pressure isgreater than the minimum temperature at which the polyarylene sulfide isfully soluble in the solvent) and 2) the upstream pressure is greaterthan the downstream pressure. Several exemplary embodiments of staticand dynamic pressure systems that meet these basic criteria aredescribed below.

According to one embodiment, the pressure differential across the filtermedium 110 can be held constant throughout the filtration process tomaintain a constant positive pressure throughout the filtration process.For instance, a constant pressure differentiation across the filtermedium 110 can be from about 50 kPa to about 400 kPa. In one embodiment,the upstream pressure can be maintained at about 800 kPa and thedownstream pressure can be maintained at about 550 kPa throughout thefiltration process with a constant pressure differential of about 250kPa throughout the filtration.

In one embodiment the pressure differential across the filter medium 110can be held constant and at a maximum differential throughout thefiltration process with the upstream pressure held at a maximum and thedownstream pressure held at a minimum so as to maximize the flow rate ofthe filtration process. In general, the maximum allowable upstreampressure can be determined based upon the filter media utilized, theconstruction materials of the filter unit, and the pressurizationcapabilities of the system. The minimum allowable downstream pressurecan be determined according to the lowest possible pressure at theoperating temperature of the filtration process such that operatingtemperature is between the boiling temperature of the mixture at thedownstream pressure and the temperature at which the polymer is fullysoluble in the solvent.

The filtration process is not limited to those embodiments in which theupstream and downstream pressures are held static throughout thefiltration, and the process can include dynamic upstream and/ordownstream pressures. For instance, the downstream pressure can becontrolled so as to maintain a positive pressure differential across thefilter media and avoid precipitation of the polymer, while both theupstream pressure and the downstream pressure can decrease over thecourse of the filtration process. According to one embodiment, thepressure differential can remain constant throughout the process, whilethe upstream pressure and the downstream pressure vary at rates similarto one another. For instance, the upstream and downstream pressures candecrease linearly and at similar rates throughout the filtrationprocess.

In a further embodiment, following a period of constant pressuredifferential between the upstream pressure and the downstream pressure,the filtration process can include a period of variable pressuredifferential between the upstream pressure and the downstream pressure.For instance, following a period of constant pressure differential whileboth the upstream pressure and the downstream pressure decline at asimilar rate, the downstream pressure can reach the minimum allowabledownstream pressure at which the boiling temperature of the mixture atthe downstream pressure can be greater than the temperature at which thepolyarylene sulfide is fully soluble in the solvent. At this point ofthe filtration process, the downstream pressure can be maintained at theminimum downstream pressure and the upstream pressure can continue todecline. The filtration process would then cease when the upstreampressure has declined to match the downstream pressure.

In yet another embodiment, the upstream or downstream pressure can bemaintained at a constant pressure throughout the process and thecorresponding downstream or upstream pressure can vary throughout theprocess. For instance, the downstream pressure can be held at theminimum downstream pressure throughout the filtration process and theupstream pressure can decrease at a linear rate from a maximum allowableupstream pressure to that point at which the upstream pressure meets thedownstream pressure, at which point the filtration process would cease.

It should be understood that a dynamic filtration process is not in anyway limited to either a decrease in upstream and/or downstream pressurethroughout all of or a part of the filtration process or to a linearchange in upstream pressure and/or downstream pressure throughout all ofor a part of the process. For instance, in one embodiment, the upstreampressure can increase throughout the filtration process and counter thereduction in flow rate that can be caused due to the formation of thefilter cake on the filter media. Moreover, the upstream pressure and/ordownstream pressure can increase or decrease in a non-linear fashion,e.g., exponentially.

In general, the pressure control design of a filtration process candepend to a large extent upon the costs involved as both the equipmentand operating costs can increase with increased operating temperaturesand pressure. Thus, one desirable design can be at the lowesttemperatures and pressures that allow the filtration process to becarried out while avoiding precipitation of the polyarylene sulfide.However, another desirable design can optimize flow rate throughincreased pressures and temperatures. In one embodiment, the system caninclude a mechanical stirrer that can agitate the filter cake during thefiltration process (for instance to maintain desired pressuredifferentials across the filter medium) and/or during a filter cake wash(to improve contact between the filter cake and the wash fluid).

Beneficially, the filtration unit and filter medium used in thefiltration process can include standard materials as are generally knownin the art. For instance, filter media as is known in the art, wiresieves or sinter plates that are stable under the filtration processconditions can be used. The mesh sizes or pore sizes of the filtermedium can be adjusted over a wide range and can vary depending on thefiltration process conditions, e.g. viscosity of the mixture, filterpressure, temperature, on the desired degree of purity of the filtrate,etc. The technical equipment as can be used for the filtration processis known, for example simple pressure filters, agitated pressurefilters, trailing blade centrifuges and rotary filters can be used,amongst others.

Following the filtration process, the filter cake can be washed toremove liquid of the filtered mixture that may remain in the filtercake. This liquid can include polyarylene sulfide in solution that canbe recovered via the wash process and as such can increase the polymeryield of the formation process. For instance, the filter cake can bewashed with solvent as is found in the mixture at a temperature at whichthe polymer will remain in solution. The resulting wash solution thatcan include dilute polymer can be combined with the filtrate of thefiltration process. In one embodiment, the filter cake can be agitatedduring the wash to improve recovery of the polymer. The washed filtercake can be dried in one embodiment to recover adhering solventresidues. The final filter cake can include very little polyarylenesulfide. For instance, the final, dried filter cake can include lessthan about 1 wt. % of the solvent by weight of the filter cake and lessthan about 1 wt. % of the polyarylene sulfide by weight of the filtercake

Following the wash, the salt cake can be removed from the filtrationmedia and the filtration media can be re-used. For instance, the saltcake can be removed from the filtration media by use of a solids port inthe system or by disassembly of the filtration unit. The salt cake canbe removed from the filtration via mechanical means (e.g., a removalblade), by a pressure differential (e.g., across the filtration media soas to ‘blow’ the filter cake off of the media), or by some combinationof methods. The salt cake can be removed in a dry or a wet state, asdesired. For example, the solid salt cake can be combined with a liquid,e.g., water, optionally in conjunction with agitation, to form a slurrywithin the filtration unit that can then be removed, for instance via asolids port in the system, and disposed of in a suitable fashion, suchas in a salt water body.

Following the first and second stages of the prepolymer formation andthe filtration process, a third stage can be carried out during which apolymerization reaction takes place and the molecular weight of thepolyarylene sulfide can be increased. During this polymerization step,water can be added to the filtrate so that the total amount of water inthe polymerization system is about 2.5 moles or less per mole of theeffective amount of the charged alkali metal sulfide. Following, thereaction mixture of the polymerization system can be heated to atemperature of from about 240° C. to about 290° C., from about 255° C.to about 280° C., or from about 260° C. to about 270° C. and thepolymerization can continue until the melt viscosity of the thus formedpolymer is raised to the desired final level. The duration of thispolymerization step can be, e.g., from about 0.5 to about 20 hours, orfrom about 1 to about 10 hours. The weight average molecular weight ofthe formed polyarylene sulfide can vary as is known, but in oneembodiment can be from about 1000 g/mol to about 500,000 g/mol, fromabout 2,000 g/mol to about 300,000 g/mol, or from about 3,000 g/mol toabout 100,000 g/mol.

Following this polymerization reaction, a second filtration process canbe carried out that can remove any additional salt from the productmixture, for instance any salt formed as the molecular weight of theprepolymer is increased during this polymerization. Alternatively, afiltration process can be carried out following this polymerizationreaction and this can be the first filtration process of the system,i.e., there need not also be a filtration process between the first andsecond polymerization processes.

Following final polymerization, the polyarylene sulfide may besolidified (as desired) and discharged from the reactor, typicallythrough an extrusion orifice fitted with a die of desired configuration,cooled, and collected. Commonly, the polyarylene sulfide may bedischarged through a perforated die to form strands that are taken up ina water bath, pelletized and dried. The polyarylene sulfide may also bein the form of a strand, granule, or powder.

Following polymerization, the polyarylene sulfide may be washed withliquid media. For instance, the polyarylene sulfide may be washed withwater, acetone, N-methyl-2-pyrrolidone, a salt solution, and/or anacidic media such as acetic acid or hydrochloric acid. The polyarylenesulfide can be washed in a sequential manner that is generally known topersons skilled in the art. The polyarylene sulfide can be subjected toa hot water washing process. The temperature of a hot water wash can beat or above about 100° C., for instance higher than about 120° C.,higher than about 150° C., or higher than about 170° C. Generally,distilled water or deionized water can be used for hot water washing. Inone embodiment, a hot water wash can be conducted by adding apredetermined amount of the polyarylene sulfide to a predeterminedamount of water and heating the mixture under stirring in a pressurevessel. By way of example, a bath ratio of up to about 200 grams ofpolyarylene sulfide per liter of water can be used. Following the hotwater wash, the polyarylene sulfide can be washed several times withwarm water, maintained at a temperature of from about 10° C. to about100° C. A wash can be carried out in an inert atmosphere to avoiddeterioration of the polymer.

Organic solvents that will not decompose the polyarylene sulfide can beused for washing the polyarylene sulfide. Organic solvents can include,without limitation, nitrogen-containing polar solvents such asN-methylpyrrolidone, dimethylformamide, dimethylacetamide,1,3-dimethylimidazolidinone, hexamethylphosphoramide, and piperazinone;sulfoxide and sulfone solvents such as dimethyl sulfoxide,dimethylsulfone, and sulfolane; ketone solvents such as acetone, methylethyl ketone, diethyl ketone, and acetophenone, ether solvents such asdiethyl ether, dipropyl ether, dioxane, and tetrahydrofuran;halogen-containing hydrocarbon solvents such as chloroform, methylenechloride, ethylene dichloride, trichloroethylene, perchloroethylene,monochloroethane, dichloroethane, tetrachloroethane, perchloroethane,and chlorobenzene; alcohol and phenol solvents such as methanol,ethanol, propanol, butanol, pentanol, ethylene glycol, propylene glycol,phenol, cresol, polyethylene glycol, and polypropylene glycol; andaromatic hydrocarbon solvents such as benzene, toluene, and xylene.Further, solvents can be used alone or as a mixture of two or morethereof.

Washing with an organic solvent can be carried out by immersing thepolyarylene sulfide in the organic solvent and heating or stirring asappropriate. The washing temperature for the organic solvent washing isnot particularly critical, and a temperature can generally be from about20° C. to about 300° C. Washing efficiency can be increased with anelevation of the washing temperature, but in general, a satisfactoryeffect is obtained at a washing temperature of from about 20° C. toabout 150° C.

In one embodiment, organic solvent washing can be combined with hotwater washing and/or warm water washing. When a high-boiling-pointorganic solvent such as N-methylpyrrolidone is used, the residualorganic solvent can be removed by washing with water or warm water afterthe organic solvent washing, and distilled water or deionized water canbe used for this washing.

The polyarylene sulfide can be utilized in forming products as aregenerally known in the art. For instance, the polyarylene sulfide can becombined with additives such as one or more of fillers (e.g., fibersand/or particulate fillers), coupling agents, an impact modifier,antimicrobials, pigments, lubricants, antioxidants, stabilizers,surfactants, waxes, flow promoters, solid solvents, and other materialsadded to enhance properties and processability. Such optional materialsmay be employed in mixture in conventional amounts.

A mixture that is melt processed to form a melt processed polyarylenesulfide composition may include a polyarylene sulfide (or a blend ofmultiple polyarylene sulfides) in an amount from about 40 wt. % to about90 wt. % by weight of the mixture, for instance from about 45% wt. % toabout 80 wt. % by weight of the mixture.

The polyarylene sulfide may be melt processed according to techniquesknown in the art. For example, the polyarylene sulfide may bemelt-kneaded in conjunction with one or more additives in a single-screwor multi-screw extruder at a temperature of from about 250° C. to about320° C. In one embodiment, the polyarylene sulfide may be melt processedin an extruder that includes multiple temperature zones. For instance,the polyarylene sulfide may be melt processed in an extruder thatincludes a temperature zone that is maintained at a temperature ofbetween about 250° C. and about 320° C.

Conventional shaping processes for forming articles including thepolyarylene sulfide may be used. For instance, extrusion, injectionmolding, blow-molding, thermoforming, foaming, compression molding,hot-stamping, fiber spinning and so forth can be used.

Shaped articles that may be formed including the polyarylene sulfide mayinclude structural and non-structural shaped parts, for instance forappliances, electrical materials, electronic products, fibrous webs, andautomotive engineering thermoplastic assemblies. Exemplary automotiveshaped plastic parts are suitable for under the hood applications,including fan shrouds, supporting members, wire and cable jacketing,covers, housings, battery pans, battery cases, ducting, electricalhousings, fuse buss housings, blow-molded containers, nonwoven or wovengeotextiles, baghouse filters, membranes, and pond liners, to name afew. Other useful articles besides moldings, extrusion and fibersinclude wall panels, overhead storage lockers, serving trays, seatbacks, cabin partitions, window covers, and electronic packaginghandling systems such as integrated circuit trays.

A composition including the polyarylene sulfide can be used in a varietyof electrical and electronic applications such as, for example,connectors and over-molding (insert-molding) parts is encompassed.

Embodiments of the present disclosure are illustrated by the followingexamples that are merely for the purpose of illustration of embodimentsand are not to be regarded as limiting the scope of the invention or themanner in which it may be practiced. Unless specifically indicatedotherwise, parts and percentages are given by weight.

Example

A 2 liter titanium pressure reactor was charged with 443.7 g of NMP,20.3 g of H₂O and 84.9 g of NaOH (96.4%). The reactor was sealed andheated to 100° C. To this mixture (SMAB-NaSH mixture) was added molten155.86 grams of NaSH (containing 71.4% NaSH and 0.7% Na₂S). The reactorwas heated to 205° C. and 75 ml of distillate containing water and NMPwas collected.

To this SMAB-NaSH mixture was added a pre-heated mixture ofpara-dichlorobenzene (p-DCB) in NMP (74% solution by weight). Thereactor was sealed and the temperature allowed to rise to 235° C. andheld for 1 hour. The temperature was then increased to 245° C. and heldfor another 3 hours for formation of the prepolymer.

Following formation of the prepolymer, the reactor was maintained at 350kPa and a filtration process as described herein utilizing a 1 L Mottfilter was carried out to remove the salt byproduct. The filtrationprocess started at 235° C. Filtration rates were 3000-5000 l/m²h afterthe filter cake had built up to about 300 mm height. The filtrate wascollected in a stirred pressure vessel that was heated to 240° C., inorder to keep the prepolymer in solution. The upstream side of thefilter was held at 90 pounds per square inch gauge (115 psia, 790 kPa)and 235° C. and the filtrate side was held at atmospheric pressure (nobackpressure 14.7 psia, 101 kPa) and about 200° C.

The salt filter cake was washed thrice with 300 grams of NMP, which waspreheated to at least 230° C. in the polymerization reactor. The washfiltrates rich in PPS were added to the first filtrate.

The filter cake was flushed with dry nitrogen, in order to pre-dry thesodium chloride. It turned out that residual moisture of less than 10weight percent was achievable within 10-15 minutes of N₂ flushing. Thisresult was reached without any mechanical means to mix or move thefilter cake, just by pressing nitrogen through the cake from thepressure side to the filtrate side. The resulting salt filter cake was apowdery, non-sticky product that allowed easy solid handling. The sodiumchloride particle size ranged from 20-60 microns in size. Theprepolymer-containing filtrate was transferred into the polymerizationreactor for further polymerization. The final stage of thepolymerization was conducted by concentrating the filtrate to 20% solidsby means of a flash distillation at 235° C. to remove all unreactedpDCB, H₂O byproduct, excess NMP and other volatiles. Following, asulfide correction was added (to increase the melt viscosity of thepolymer from <1 Poise to a final melt viscosity of 460 Poise). Thepolymerization was conducted by heating the reactor to 245° C. andholding the temperature for 1 hour then raising the temperature to 260°C. and holding it for 3 hours. After the hold time, 90 grams of waterwas added via a pump and the pressure of the reactor was increased from100 psi up to 270 psi. After the addition, the temperature was allowedto cool down to obtain a granular PPS polymer. To isolate the PPS, theslurry was filtered and washed with once with 1 L NMP followed by 1 L of3% acetic acid in water at 60° C. then 3 times with 80° C. water. Thewashed polymer was dried at 104° C. vacuum oven. The yield was 200 g(93% of theory).

The filter media following the filtration for this run is illustrated inFIG. 2. In this sample, the downstream pressure (atmospheric)corresponded to a boiling temperature for the filtrate that was lowerthan the temperature at which the polyphenylene sulfide was fullysoluble in the NMP. This caused the polyphenylene sulfide to precipitateand plug the filter, as can be seen in FIG. 2.

Example 2

The process of Example 1 was repeated, but in this Example, the upstreamside of the filter was held at 90 pounds per square inch gauge (115psia, 790 kPa) and 235° C. and the filtrate side was held at 65 poundsper square inch gauge (80 psia, 550 kPa) and 235° C. The filter mediafollowing the filtration for this run is illustrated in FIG. 3. In thissample, the downstream pressure corresponded to a boiling temperaturefor the filtrate that was higher than the temperature at which thepolyphenylene sulfide was fully soluble in the NMP. As such, there wasno precipitation or plugging of the filter, as can be seen in FIG. 3.

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications maybe made therein without departing from the scope of the subjectinvention.

What is claimed is:
 1. A method for forming a polyarylene sulfidecomprising: reacting a dihaloaromatic compound with an alkali metalsulfide or an alkali metal hydrosulfide in an organic amide solvent toform a polyarylene sulfide and a salt; subjecting a mixture includingthe polyarylene sulfide, the salt, and the organic amide solvent to afiltration process in which the mixture flows to a filter medium fromupstream of the filter medium and in which a filtrate flows away fromthe filter medium in a downstream direction, the salt being retained onthe filter medium during the filtration process and forming a filtercake, the filtration process having a downstream pressure, thedownstream pressure being elevated above atmospheric pressure, theboiling temperature of the mixture at the downstream pressure beinggreater than the minimum temperature at which the polyarylene sulfide isfully soluble in the solvent, the filtration having an upstreampressure, the upstream pressure being greater than the downstreampressure for at least a portion of the filtration process, thefiltration being carried out in a temperature range that is less thanthe boiling temperature of the mixture at the downstream pressure andthat is greater than the minimum temperature at which the polyarylenesulfide is fully soluble in the solvent.
 2. The method of claim 1,further comprising heating the mixture and the filtrate during thefiltration process.
 3. The method of claim 2, wherein the mixture andthe filtrate are independently heated to temperatures of from about 220°C. to about 300° C.
 4. The method of claim 1, wherein the downstreampressure is greater than about 300 kilopascals.
 5. The method of claim1, wherein the difference between the upstream pressure and thedownstream pressure is from 30 kilopascals to about 500 kilopascals forat least a portion of the filtration process.
 6. The method of claim 1,wherein the difference between the upstream pressure and the downstreampressure is constant throughout at least a portion of the filtrationprocess.
 7. The method of claim 6, wherein the constant difference isfrom about 50 kilopascals to about 400 kilopascals.
 8. The method ofclaim 6, wherein the constant difference is the maximum allowabledifference.
 9. The method of claim 6, wherein the upstream pressure isconstant throughout the filtration process and the downstream pressureis constant throughout the at least a portion of the filtration process.10. The method of claim 6, wherein both the upstream pressure and thedownstream pressure vary during the at least a portion of the filtrationprocess.
 11. The method of claim 1, wherein the difference between theupstream pressure and the downstream pressure varies during at least aportion of the filtration process.
 12. The method of claim 11, whereinone of the upstream pressure and the downstream pressure variesthroughout the filtration process.
 13. The method of claim 11, whereinboth the upstream pressure and the downstream pressure vary throughoutthe filtration process.
 14. The method of claim 1, wherein the alkalimetal is sodium.
 15. The method of claim 1, wherein the dihaloaromaticcompound is dichlorobenzene.
 16. The method of claim 15, wherein thedichlorobenzene is m-dichlorobenzene, o-dichlorobenzene,p-dichlorobenzene, or mixtures thereof.
 17. The method of claim 1,wherein the organic amide solvent is N-methylpyrrolidone.
 18. The methodof claim 1, wherein the polyarylene sulfide is a homopolymer or acopolymer.
 19. The method of claim 1, wherein the polyarylene sulfide islinear.
 20. The method of claim 1, wherein the polymerization reactionforms a low molecular weight prepolymer.
 21. The method of claim 20,wherein the polymerization reaction conversion rate of thedihaloaromatic compound is not less than about 50% of the theoreticalconversion rate.
 22. The method of claim 20, wherein the weight averagemolecular weight of the low molecular weight prepolymer is from about500 grams per mole to about 30,000 grams per mole.
 23. The method ofclaim 20, further comprising a second polymerization reaction.
 24. Themethod of claim 23, wherein the second polymerization reaction takesplace following the filtration process.
 25. The method of claim 23,wherein the second polymerization reaction takes place prior to thefiltration process.
 26. The method of claim 23, wherein the weightaverage molecular weight of the polyarylene sulfide following the secondpolymerization reaction is from about 1,000 grams/mole to about 500,000grams per mole.
 27. The method of claim 24, further comprising carryingout a second filtration process following the second polymerization. 28.The method of claim 1, further comprising washing the filter cakefollowing the filtration process.
 29. The method of claim 28, whereinthe wash solution from the washing step is combined with the filtrate.30. The method of claim 28, wherein the washing step includes agitationof the filter cake.
 31. The method of claim 1, further comprising dryingthe filter cake.
 32. The method of claim 1, further comprising combiningthe filter cake with a liquid to form a slurry.
 33. The method of claim1, further comprising removing the filter cake from the filter mediumand re-using the filter medium.
 34. The method of claim 1, furthercomprising combining the polyarylene sulfide with one or more additives.35. The method of claim 1, further comprising shaping the polyarylenesulfide to form a product.
 36. A system for carrying out the method ofclaim 1, the system including a reactor and a filter unit, thepolyarylene sulfide being formed in the reactor and the filtrationprocess being carried out in the filter unit.
 37. The system of claim36, further comprising a second reactor downstream of the filter unit.38. The system of claim 36, further comprising a second reactor upstreamof the filter unit.
 39. The system of claim 36, wherein at least aportion of the filter unit is heated.
 40. The system of claim 39,wherein at least that portion of the filter unit that contacts themixture and the filtrate during the filtration process is heated.